page_title: Jellyfish Lake
section_title: Golden jellyfish
hierarchical_section_title: Jellyfish Lake / Quotidian migration / Golden jellyfish
caption_attribution_description: English: map of Eil Malk or Mecherchar Island, Rock Islands, Palau, West Pacific Ocean, (topographic) 1:25,000
context_page_description: Jellyfish Lake is a marine lake located on Eil Malk island in Palau. Eil Malk is part of the Rock Islands, a group of small, rocky, mostly uninhabited islands in Palau's Southern Lagoon, between Koror and Peleliu. There are about 70 other marine lakes located throughout the Rock Islands. Millions of golden jellyfish migrate horizontally across the lake daily.
  Jellyfish Lake is connected to the ocean through fissures and tunnels in the limestone of an ancient Miocene reef. However the lake is sufficiently isolated and the conditions are different enough that the diversity of species in the lake is greatly reduced from the nearby lagoon. The golden jellyfish, Mastigias cf. papua etpisoni, and possibly other species in the lake have evolved to be substantially different from their close relatives living in the nearby lagoons.
context_section_description: The golden jellyfish migration pattern is as follows:
  Night - For about 14 hours a day the jellyfish make repeated vertical excursions between the surface and the chemocline in the western basin possibly to acquire nitrogen and other nutrients from near the chemocline for their symbiotic algae.
  From early morning to about 0930 - The jellyfish move from center of western basin to the eastern basin
  From early afternoon to about 1530 - The jellyfish move from eastern basin to near western end of lake
  As the sun sets - The jellyfish move briefly eastward from western end to western basin where they remain through the night
  The golden jellyfish rotate counter-clockwise as they swim at the surface, presumably to provide even exposure to the sun for the symbiotic algae in their bodies.
  The Jellyfish Lake golden jellyfish migration pattern is similar to Mastigias sp. in other Palauan marine lakes and coves which all migrate west to east in the morning. However the migration patterns in other Palauan coves and marine lakes are less well defined than that of the Jellyfish Lake golden jellyfish. The east to west migration in all of these other lakes (except for Clear Lake on Eil Malk) does not begin until the late afternoon.
  Hamner and Dawson proposed that the difference is caused by evolutionary change driven by the jellyfish-eating anemones (Entacmaea medusivora) that inhabit the eastern regions of Jellyfish Lake and Clear Lake. The jellyfish instinctively avoid shadows and in the morning with the shadows on the eastern end the jellyfish also avoid the anemones. By moving east to west in the early afternoon the jellyfish avoid the time of day when the setting sun would eliminate shadows on the lake in the eastern end and thereby avoid the anemones in the afternoon.
caption: Map of Eil Malk with Jellyfish Lake in the eastern part of the main island
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